Accurate interpretation of X-ray film images is crucial in medical diagnostics. Radiographic images in the form of X-ray transparencies are used to provide a high-resolution, wide dynamic range, and low cost representation of internal anatomical structure. Furthermore, the transparencies form permanent records, radiographic technology is well understood, and a large body of trained practioners can read transparencies quickly and accurately.
X-ray film has several disadvantages. Once an image has been developed on an X-ray transparency, virtually no adaptive processing can be performed on the transparency, for example, to manipulate greyscales to highlight desired areas. Re-exposure to obtain better, or different, views is undesirable in view of cost, time, and health considerations. The diagnostician, therefore, must make optimum use of a limited number of X-ray exposures when analyzing the condition of a patient.
The accuracy of reading X-ray transparencies depends upon the quality of illumination used to view the transparencies. Currently, a transparency is read by placing it on a lightbox consisting of a diffusion screen which is uniformly lit from the rear. Such lightboxes frequently cover a wall in a viewing room, and practitioners move transparencies about so that different exposures from a study can be compared with each other, or even with exposures taken from other modalities (such as an ultrasonic apparatus).
Transparency interpretation by means of a lightbox supports a simple and inexpensive diagnostic procedure, but one which suffers from a number of inadequacies related to the wide range of intensities transmitted through the transparency to the viewer. The optical transmission at each point of an X-ray transparency is measured in optical density (O.D.) units. One O.D. unit corresponds to a reduction of light intensity by a factor of 10 as the light passes through a transparency. Two O.D. units corresponds to a factor of 100 reduction in light intensity. A typical X-ray transparency may contain a range of recorded optical densities of up to 2 or 3, or even more. This means that light intensities that range over a factor of 100 to 1000 or more will be encountered when viewing a typical transparency. In the presence of such a wide dynamic range of light intensities, the human eye is unable to discriminate between all of the subtle differences inherent in the low contrast detail of film densities that must be detected in order to convey information recorded on the film to the mind of the practitioner.
Normally, the practitioner deals with the dynamic range problem inherent in X-ray transparency in a number of ways. In regions of extreme darkness, the practitioner can place the dark region of the film in front of a small bright light source in order to highlight the low contrast detail shrouded in darkness. Alternatively, the practitioner can restrict the visual field to a small region of film by looking through a long narrow tube, such as a rolled-up paper sheet. Such a tube has the effect of limiting the range of light intensities only to that occurring in the viewed region. If the range is smaller than the dynamic range present on the entire film, the practitioner will be better able to detect detail in the low frequency variation of the viewed region.
Both of these techniques simplify the visual tasks of the viewer by reducing the dynamic range of light intensities that must be viewed simultaneously. The reduced dynamic range is better matched to the capabilities of the human eye. However, these techniques, and other nonsystematic approaches to adapting viewing conditions for film interpretation are cumbersome, time consuming to implement, and are only partially effective. For all of these reasons, medical diagnosis is frequently based upon interpretation under suboptimal viewing conditions.
A better approach to this problem would be to employ a fast, automated, adaptive system of image processing that would reduce the dynamic range of light intensities presented by a given transparency to a viewer, without sacrificing the viewer's ability to detect the transparency's low contrast detail vital to accurate interpretation.
At this time, one principal mode for adaptively changing the dynamic range of an X-ray transparency image is to digitize the original image at the highest possible resolution (at least 1024.times.1024 pixels and up to 12 bits of greyscale) and then manipulate this representation in an image processor, and view the results on either hard copy or on a high-resolution monitor. However, these processes can be long, taking several minutes for enhancements. Further, they may alter the diagnostic environment from the familiar one of transparency viewing to an unfamiliar one of hardcopy or monitor viewing. Also a CRT monitor does not have the dynamic range of contrast that can be achieved with film. To a certain extent, this deficiency can be overcome by adaptive mapping of image contrast levels and by other techniques such as pseudo-color enhancement. However, this spatial resolution of a video monitor is limited and some image information may be lost. Although this can be alleviated somewhat by zooming and windowing of an image, the whole image cannot be seen in high resolution at once.
Therefore a primary objective of the invention is to provide for enhancement of transparency image viewing by adaptation of the transparency viewing process, rather than by substitution of another viewing process.
Another objective is to enhance the visibility of low-contrast detail in the image on an X-ray transparency through adaptation of the structure of a conventional viewing apparatus, such as a lightbox.